Italian police say they have arrested four Moroccans who were planning a chemical attack in Rome, targeting buildings which included the United States embassy.

We will be discussing the matter with Washington today

US Embassy spokeswoman

The four men, aged between 30 and 40, were detained on Tuesday in a dawn raid on a flat in the Tor Bella Monaca area in the south of the Italian capital. The news did not emerge until early on Wednesday.

They were discovered to be in possession of a powdered cyanide-based substance, maps marking the capital's water supply network, as well as a hoard of false documentation and Islamic extremist propaganda, officials said.

It is suspected that they were intending to poison the water supplies in a commercial area of the capital where the US embassy is located.

"The speculation is that we were a target," a spokeswoman at the embassy in Rome told BBC News Online. "We'll be discussing the matter with Washington today."

The four kilos of the substance in the possession of the four Moroccans is reported to be sufficient to poison the water supply of an entire neighbourhood, and cause dozens of deaths.

Terror links

The BBC's Frances Kennedy in Rome says there is mounting speculation that the men are linked to Osama Bin Laden's terrorist al-Qaeda network.

Police said the men have been under surveillance for several days. Officers moved in to arrest them once they had sufficient evidence they were in possession of a potentially deadly substance.

The tapping of their telephones has also allegedly revealed links between the four Moroccans and four Tunisians currently on trial in Milan over purported ties with the al-Qaeda network.

They are accused of conspiring to traffic in false documents, weapons, explosives and chemical weapons.

But Italian magistrates are said to be furious that the news of the arrests of the Moroccans has been leaked to the press.

They say it will jeopardise efforts to reveal who the men were working with, and who had supplied them with the poisonous substance.